1. Field of the Invention
The present invention relates to a process for manufacturing low-dusting, smoothly-discharging, easily dispersible titanium dioxide and other pigments and cohesive powders that resist compaction, aging, lumping and caking, in which the pigment or powder has been subjected to micronizing or jet milling, sand milling, hammer milling, ground up or the like for use in foodstuffs, cosmetics, detergents, paint and plastics, inks and elastomers.
2. Description of the Prior Art
Titanium dioxide, iron oxides pigments, pearlescent pigments and powders including talc, and other metal oxide pigments, are used in the cosmetics, detergents paint, plastics, construction and other industries where pigments or powders are added to color and /or opacify the desired application and usually through intensive mixing. Some essential performance properties for the pigments are the dispersability of the pigment throughout the application system, the ease of handling and metering and dusting.
Dispersability is a measure of the ease with which the powder or pigment particles can be uniformly and intimately mixed in a system. Poor dispersion of particles can cause large agglomerates that may result in lumps, surface imperfections, color streaks, non-uniform coloration, or incomplete color development within the product. A further problem exists for pelletized or granular powder or pigment in that the amount of energy expended to disperse those types is exceptionally high and time consuming.
Inorganic pigments are normally produced for and used in the paints, plastics, or elastomer industries, and are in the form of a finely divided powder. The powders are usually jet-milled, sand milled, hammer milled or roller milled as a finishing step in their production. Milling contributes to dispersability and gloss, but milled pigments exhibit poor dry flow characteristics and are dusty. Such pigments all have the great disadvantage of producing dust. During their use, costly measures (for example of a workplace safety, ecological or quality assurance nature) are necessary, and valuable material is lost.
The ease of handling or lack thereof, takes into consideration difficulties associated with storing, transportation and mixing of the pigments in the manufacturing and processing system. The stability of pigment granules is necessary for good storage and transporting, thus averting aging, or the clumping of pigment into large agglomerates when in storage subjected to heat, humidity and pressure. This stability is brought about by using the high cohesive forces within the individual particle to an advantage rather than a disadvantage and depends on the compaction pressure or forming method used in making the granules. It should be noted that the goals of achieving good dispersability and good stability are usually contrary to one another.
Problems repeatedly experienced in handling large quantities of powders are caking, rat holing, bridging, or aging in compressed storage and clogging with a loss of pigment flow when in feed bins. Combined with the problems associated with dust from finely divided powders it is frequently the case that pigment compositions are preferred in pellet particle or granular form.
Surface treatment of pigments to achieve improved performance characteristics such as dispersion in coatings and plastic compositions, and may help in dry flow, is known in the art as is shown in the following patents:
U.S. Pat. No. 3,925,095 teaches that treating of inorganic pigments or fillers with hydroxyalkylated alkylene diamine dispersing agent will help flowability and dispersability in many applications.
U.S. Pat. No. 4,127,421 discloses the aqueous treatment of a lead chromate-containing pigment with a friable hydrocarbon resin and a cationic surfactant will produce free flowing, non-dusting granules.
Also known in the art is the absorption of waxes, aqueous solutions, polymers, and other oils, fats, or surfactants to help form free flowing granules.
In U.S. Pat. No. 4,285,994 the invention comprises a free-flowing nucleated pigment 50% by weight or more, in a spherical particulate of spray-chilled wax composition that has a coating and is partially absorbed into a pigment.
In U.S. Pat. No. 4,056,402, shows that aqueous slurries of inorganic or organic pigments are milled in the presence of a nonionic, polyether alcohol dispersing agent and nonionic cellulose ether to reduce dusting and improve dispersability.
U.S. Pat. No. 4,310,483 demonstrates a process for producing a granulate of fusible additives and pigments for plastics by thermal tumbling granulation, that comprises mixing the additive in powder form with an additive that softens between 30 degree C. and 200 degree C. and which have a particle size of 0.1-2.0 mm.U.S. Pat. No. 4,375,520. This patent teaches the production of dustless particles, including pigments, by treatment of the particles with a solid low-molecular weight polymer and a liquid polymer substance such as expoxidized soybean oil at temperatures above the melting point of the polymer.U.S. Pat. No. 4,464,203 discloses the treatment of inorganic or organic pigments with an amine and ethylene oxide block copolymer surfactant to improve pigment dispersibility and other properties in many applicationsU.S. Pat. No. 4,762,523 discloses coating a moist pigment with a polyester surfactant, then adding mineral oil or wax to the pigment and applying high shear stresses to achieve a free-flowing, permanently non-dusting pigment.U.S. Pat. No. 5,604,279 shows that a colorant composition comprising at least one colorant very finely dispersed in a base material which is solid at room temperature is suitable for producing master batches and provides the way to dust-free colorant compositions
One method for making free-flowing powders with low dust can be obtained by spray drying. These products generally exhibit poor pigmentary properties. Thus, pigment end users have generally had to choose between free-flowing, low dusting, spray-dried pigments with poor pigmentary properties and dusty, milled pigments with poor flow characteristics.
U.S. Pat. No. 3,660,129 establishes that titanium dioxide pigments are coated with hydrous oxides, then sand-milled and spray-dried to improve flowability.
U.S. Pat. Nos. (4,810,305) and (5,035,748) represent spray pelletization using siloxanes as hydrophobic additives. The stated spray dryer generally results in small particle sizes with a high proportion of fines. This means that a substantial proportion of the material is not realized from the dryer as directly usable pellets, but as fines, which must first be retained in a filter and then returned to the process. In spray dried products, hydrophobic post-treatment results in particles that have somewhat good flow properties but produce exceptionally large quantities of dust.U.S. Pat. No. 5,199,986 demonstrates that colored building materials are produced by incorporating into the building materials inorganic pigments in the form of granulates which are free-flowing and not dust forming where the pigments are produced from spray dried granulates by post-granulatingU.S. Pat. No. 5,199,986 also discloses a process wherein previously spray-dried inorganic pigment granules are coated with water and solutions of salts of boron, aluminum, silicon, titanium, zinc, brass, or tin, to improve processing and reduce dust production.U.S. Pat. Nos. 5,733,365 and 5,908,498 illustrate a process for manufacturing titanium dioxide characterized by improved flowability, low dust production, and good dispersibility, without the energy-intensive and expensive step of micronization. In this process, at least one treating agent is deposited on a pigment that has been sand milled, as an aqueous slurry. The treated slurry is then spray dried for end-use performance without jet milling or micronization.
Known production processes for pigment pellets in addition to the spray granulation (spray drying using a disk or jet) are agglomeration pelletization (mixers, fluidized bed pelletizers, plates or pins) or compaction processes.
U.S. Pat. No. 6,132,505 instructs that inorganic pigment pellets comprising the steps of mixing at least one inorganic pigment powder with at least one water-soluble, hydrophilic or hydrophobic/hydrophilic auxiliary substance liquid in a quantity of 0.1 to 10 wt. %, relative to the pigment powder, and pelletizing the resultant mixture of by compacting and crushing the resultant mixture, one fraction having an average particle size of 100 to 1000 μm.
None of the aforementioned patents enlighten, teach or even suggest about a process consisting of mixing of pigments with the presence an air flow and an electrostatic charge to obtain a pseudo particle to provide an extremely friable, highly dispersible pigment that is smoothly discharging, with reduced dusting, resistant to compaction and easily metered, and this is the subject of this application.